Apolipoprotein D (apoD), a member of the lipocalin superfamily of transporters has been implicated in neurodegenerative disorders, neural injury and in neural regeneration. For instance, in Alzheimer's disease, there is increased apoD in the entorhinal cortex and in spinal fluid and the increased apoD correlates with inheritance of the apoE4 genotype. In the lysosomal cholesterol storage disorder, Niemann-Pick type C disease (NP-C), there is an induction of the apoD gene and apoD protein levels are increased several-fold in the brain and in plasma. In the regenerating sciatic nerve in the rat, following a crush injury, there is an almost 500-fold increase in apoD levels. ApoD mRNA and protein levels have also been shown to be upregulated following injury to the central nervous system. Finally, apoD gene expression has been shown to be increased in response to atypical neuroleptics suggesting that it may be a modulator of neuronal signal transduction. At a cellular level, we have found that apoD and NPC1, the product of the gene defective in most cases of NP-C, are components of a vesicular trafficking pathway that directs the retroendocytic movement of cholesterol and glycolipids. NPC1 is enriched in astrocytic foot processes around neuronal synapses. ApoD, on the other hand, is present in oligodendrocyte precursor cells, in pericytes, and in perivascular fibroblasts. In cultured peripheral cells and in glia, apoD and NPC1 localize to endocytic vesicles in a cholesterol- and glycolipid-dependent manner. The proposed studies aim to investigate further the role of apoD in neurodegeneration. To enable these studies we will use apoD knockout, apoD transgenic (expressing the human apoD gene) and apoD/NPC1 double knockout mice to investigate the following specific aims: (1) the developmental, behavioral and neurological phenotypes of apoD knockout, apoD transgenic and apod/NPC1 double knockout mice, and their response to neurotoxic and neurodegenerative lesions (2) cholesterol trafficking in apoD knockout and apoD/NPC1 double knockout mice (3) functional interaction(s) of apoD and NPC1 proteins using fluorescence resonance energy transfer (FRET), and (4) apoD and NPC1-mediated regulation of glial and neuronal cholesterol homeostasis.